1. Field of the Invention
The present invention relates to an optical information recording medium to which and from which recording and reproduction can be performed by irradiating a short-wavelength laser beam.
2. Description of the Related Art
An example of an optical information recording medium in which an organic dye is used as a recording material is a write-once optical information recording medium. This is an optical information recording medium in which information has been recorded, and the recorded information is reproduced by irradiating a laser beam emitted from a semiconductor laser having a wavelength in the range of 750 to 830 nm, i.e., a red laser beam. First, write-once CD (i.e., CD-Recordable (CD-R)) discs having a recording capacity of 650 MB or 700 MB were developed. Subsequently, write-once DVD (i.e., DVD-R/+R) discs having a recording capacity of 4.7 GB were developed for which a short-wavelength red laser beam is used, the laser beam having a wavelength in the range of 640 to 680 nm, which is shorter than the above wavelength. These CD-R discs and DVD-R/+R discs have been widely used. The recording polarity of these CD-R discs and DVD-R/+R discs is of a high-to-low type (hereinafter referred to as “HTL type”). More specifically, in this HTL type, an unrecorded portion where a pit is to be formed has a high reflectance, and the pit portion formed after recording has a low reflectance.
As an optical information recording medium having a larger capacity, optical discs have been developed to which and from which recording and reproduction can be performed using a blue-violet laser beam having a wavelength in the range of about 350 to 500 nm (for example, about 405 nm), which is shorter than the wavelength of the conventional short-wavelength red laser beam. In 2006, organic HD DVD-R discs including an optical recording layer made of an organic dye were commercialized. These organic HD DVD-R discs have a recording capacity of 15 GB in a single layer and recording into and reproduction from the organic HD DVD-R discs are performed using a laser beam having a wavelength of 405 nm. Furthermore, write-once inorganic blu-ray discs (i.e., inorganic BD-R) including an optical recording layer made of an inorganic material were also commercialized, and the recording capacity in a single layer thereof is further increased and recording and reproduction are performed using a laser beam having a wavelength of 405 nm. In these organic HD DVD-R discs and inorganic BD-R discs, recording is performed along a groove as in the DVD-R/+R discs. The recording polarity of the organic HD DVD-R discs is of a low-to-high type (hereinafter referred to as “LTH type”). More specifically, in this LTH type, an unrecorded portion where a pit is to be formed has a low reflectance, and the pit portion formed after recording has a high reflectance. On the other hand, the recording polarity of the write-once inorganic blu-ray discs is of the HTL type as in the DVD-R/+R discs.
As shown in FIG. 1, an HD DVD-R disc 1 that has already been commercialized includes a disc-shaped substrate 2 having a groove 7 serving as a guiding groove on a principal surface thereof. A land 8 is provided between a pair of concentric sections of the groove 7 of the substrate 2. An optical recording layer 3 is formed on the upper surface of the disc-shaped substrate 2 having the groove 7 thereon. A light-reflecting layer 4 is formed on the optical recording layer 3. A dummy substrate 6, which also functions as a protective layer, is laminated on the upper surface of the light-reflecting layer 4, with an adhesive layer 5 therebetween. In the HD DVD-R disc 1 thus obtained, recording is performed by irradiating a laser beam 9 onto the optical recording layer 3 along the groove 7, which is a spiral guiding groove provided on the substrate 2. Reproduction from the HD DVD-R disc 1 into which recording has been performed along the groove 7 is performed by irradiating a laser beam 9 along the groove 7 and reading the recorded signals from the reflected light of the laser beam 9. That is, recording and reproduction are performed along the groove 7. The recording polarity of this HD DVD-R disc 1 is of the LTH type in which an unrecorded portion where a pit is to be formed has a low reflectance and the pit portion formed after recording has a high reflectance.
For example, according to Japanese Unexamined Patent Application Publication No. 2005-297406, in order to realize a high-definition HD DVD-R disc in a wavelength range comparable to that of a short-wavelength laser beam using an organic dye for an optical recording layer, the optical recording layer is formed using an organic dye which has an anion portion and a cation portion and whose maximum absorption wavelength range is present at the longer wavelength side of the wavelength of a short-wavelength laser beam to be irradiated onto the optical recording layer.
FIG. 2 shows a write-once organic blu-ray disc (hereinafter referred to as “organic BD-R”) in which an organic dye is used for an optical recording layer. In an organic BD-R disc 11, a light-reflecting layer 4, an optical recording layer 3, and a protective layer 5 are sequentially formed on a surface of a disc-shaped substrate 2, the surface having a guiding groove thereon. Furthermore, a cover layer 6b is bonded to the upper surface of the protective layer 5, with an adhesive layer 6a therebetween. In this organic BD-R disc 11, the spiral guiding groove provided on the substrate 2 functions as a groove 7′, and a portion disposed between a pair of concentric sections of the groove 7′ is a land 8′. Recording is performed by irradiating a laser beam 9 onto the optical recording layer 3 in the groove 7′ or the optical recording layer 3 on the land 8′. Reproduction from the organic BD-R disc 11 into which recording has been performed along the groove 7′ or along the land 8′ is performed by irradiating a laser beam 9 along the groove 7′ or land 8′ and reading the recorded signals from the reflected light of the laser beam 9. For example, in order to realize a high-definition organic BD-R disc in a wavelength range comparable to that of a short-wavelength laser beam using an organic dye for an optical recording layer, Japanese Unexamined Patent Application Publication No. 2003-303442 (document '442) discloses an invention in which the ranges of the refractive index (n) of the optical recording layer before recording, the attenuation coefficient (k) of the optical recording layer before recording, and the amounts of change in n and k before recording and after recording, i.e., Δn and Δk, are specified.
However, the recording polarity of the organic BD-R disc described in document '442 is of the HTL type in which an unrecorded portion where a pit is to be formed has a high reflectance and the pit portion formed after recording has a low reflectance. In an organic BD-R disc including an optical recording layer made of an organic dye, in order to improve the recording sensitivity, a large attenuation coefficient (k) of the optical recording layer before recording is advantageous because a large amount of a laser beam is absorbed. However, such a large attenuation coefficient (k) before recording cannot be realized, which is a problem to be solved.